This invention relates to planar balanced circuits and more particularly to a single balanced broadband millimeter-wave mixer covering frequency ranges up to full waveguide bands. Standard waveguide techniques are not suitable to satisfy the need for broadband millimeter-wave mixers covering frequency ranges in these bands. Particularly, balanced mixers, which are desirable in most systems applications, have limited bandwidth if short-slot or magic-Tee hybrids are employed. In addition, conventional waveguide structures are physically large and often costly to produce. Until recently, two major obstacles limited the bandwidths of waveguide mixers. For many years, chip diodes mounted in a so-called Sharpless waveguide wafer were the only diodes suitable for millimeter-wave frequencies. These diodes were limited in bandwidth because of the parasitic reactances of the chip itself and those of the wafer mount. More recently, GaAs Schottky barrier diodes with very high cutoff frequencies (1,000 GHz and higher) and low junction capacitances have become available. Even more important, GaAs diodes are now on the market in the form of beam-lead devices which make microwave integrated circuits (MIC) at millimeter-wave frequencies practical. Because the MIC transmission-line geometries such as microstrip or strip-line are smaller than waveguide geometries, and the electromagnetic fields are confined to smaller areas, interconnections with small semiconductor diodes are now possible with greatly reduced parasitic reactances.
For wide band applications, single ended mixers are normally insufficient because they lack local-oscillator (LO) noise suppression and low spurious response density. Therefore, balanced or double balanced mixers have to be used. Balanced mixers require either a 90.degree. or 180.degree. hybrid to combine the LO signal and RF signal. Waveguide hybrids are not practical for wide band applications because of their limited bandwidth as in the short-slot coupler, the branch-line coupler and the magic-Tee and because of excessive coupling imbalances as in the multi-hole coupler. In addition, all waveguide couplers to date are quite large in size and require two separate transitions to a circuit medium convenient to mount the two diodes. On the other hand, strip-line or microstrip wideband quadrature hybrids from 26.5 to 40 GHz are very difficult to realize. Branch-line couplers covering the complete Ka-band are feasible but their performance is not very appealing.
It is important at this point to note the differences between a mixer using a 90.degree. hybrid and one using a 180.degree. hybrid. The 90.degree. hybrid generally has good LO and RF port VSWRs but suffers from a low LO-to-RF isolation. Opposed to that, a 180.degree. hybrid-coupled mixer has good LO-to-RF isolation but has normally poor LO and RF port VSWRs.